The present invention relates to a method and a device for driving a supersonic motor.
Conventionally, a general method of driving a supersonic motor was to apply a signal of a resonant frequency of the supersonic motor to the supersonic motor (Japanese Patent Application No. 62-85684 and Japanese Patent Application No. 62-92781). Japanese Patent Application No. 62-85684 discloses a method of driving the supersonic motor, which comprises; providing monitor electrodes for detecting a driving condition of the supersonic motor in a piezoelectric element, detecting a phase difference between a monitor signal generating from the monitor electrodes and a driving frequency voltage applied to the piezoelectric element to drive the supersonic motor and determining a frequency of the frequency voltage so that the phase difference becomes a phase difference in a resonant condition of the supersonic motor.
Japanese Patent Application No. 62-92781 discloses a method of driving the supersonic motor near a resonant frequency of an elastic body, which comprises; dividing an output variable oscillator having the resonant frequency of the elastic body in a variable range into two, amplifying one output after it is shifted by 90.degree. and applying the output to the piezoelectric element while amplifying the other output directly and applying the output to the piezoelectric element, comparing a voltage waveform phase and a current waveform phase of either of the two outputs, and adjusting the phase difference below a set level.
However, the prior art discussed above for example, created a problem. The problem is the supersonic motor generates an audible sound when the supersonic motor is driven in the resonant frequency.
Japanese Patent Application No. 62-203575 discloses a supersonic motor controlling an input power source frequency so that a monitor voltage becomes a constant voltage which is lower than a maximum monitor voltage obtained in the resonant frequency in a frequency range which is higher the resonant frequency of a stator comprising an elastic body and a piezoelectric element. However, Japanese Patent Application No. 62-203575 created another problem that the supersonic motor generates the audible sound because a driving frequency cannot be followed enough according to a change in load by a driving frequency follow method using the constant monitor voltage. The prior art created a further problem that the driving frequency cannot change according to a change of the motor characteristics due to a change in temperature and load so that the supersonic motor cannot output enough because the supersonic motor keeps the monitor voltage becoming the constant voltage. The prior art created another further problem that when the supersonic motor is locked, the driving frequency deviates from a tolerance, driving the supersonic motor below the resonant frequency, which possibly leads a power amplifier of the driving device to burn. Thus, a merit of the supersonic motor that the motor makes a low noise and is small in size and light in weight could not be put to a practical use efficiently in case that the supersonic motor is employed for a vehicle.